Abstract: Axial compressive experiments were carried out up to fracture and extended to frictional sliding for three types of initially water-saturated cylindrical rock samples (gabbro,graniteⅠ and Ⅱsaturated with tap water or salt water respectively) at 1.2kb confine pressure. Electrical resistivities of rock samples were measured during all these experiments.On the basis of Brace's method of investigating surface conduction,we analysed the experimental results by introducing two special terms namely the volume resistance coefficient and the surface conduction coefficient for a given wet rock sample. We found that all surface conduction coefficient increased wfth deviatoric stress as soon as the rock sample began to dilate. It was proved that resistivities in wet rock 'samples decrease after dilatancy mainly because of the increase of the surface conductivity.In contrast to previous work,the resistivity of various rock samples changed in different ways during frictional sliding. It is proposed in this paper that this difference may be related to the size of grains in the rocks. The resistivity of rock sample containing coarser grains decreases whereas that of rock sample containig finer grains increases during sliding.